Shaft for an electric motor with DC brake

ABSTRACT

A motor with DC brake having a DC electromagnetic brake (10) arranged on the shaft (14) of the rotor (16) thereof, in which a partial shaft (14b) extending through and outward from the DC electromagnetic brake (10) is formed of a nonmagnetic metal material. A partial shaft (14a) extending through the rotor (16) is formed of a free cutting metal material, and partial shafts (14a) and (14b) are coaxially united to form the integral shaft (14) at a junction (18) by friction welding.

TECHNICAL FIELD

The present invention relates to an electric motor with a DC brakehaving a brake mechanism which is disengaged upon being excited by adirect current, more particularly to an improvement of the shaftstructure of the motor.

BACKGROUND ART

A motor with built-in brake having a brake mechanism associated with therotor shaft has conventionally been used. The brake mechanism of a motorwith a built-in brake is constituted so that the rotation of the rotoris braked by pressing a brake pad with a spring pressure against a brakedisk secured to the rotor shaft. The rotor is released by disengagingthe brake pad from the brake disk through the excitation of a DCelectromagnet. Such a brake mechanism includes a brake disk, a brakepad, a braking spring or springs, and a DC electromagnet for disengagingthe brake is disposed coaxially with the rotor within a case at thefront end or at the rear end of the motor. In such a brake mechanism,the DC electromagnet is disposed near the rotor shaft. Therefore, aleakage flux flows as far as to the part of the rotor shaft extendingoutside the motor when the rotor shaft is formed of a material, such asa carbon steel, which is easily workable and has high mechanicalstrength and magnetic conductivity. Consequently, when such an electricmotor with a DC brake is employed as the driving source of a machinetool, powder of magnetic materials, such as iron powder, adheres to theprojecting part of the rotor shaft, adversely affecting the mechanicalconnection between the projecting part of the rotor shaft and theassociated driven body. When a revolution detector is provided on therotor shaft of the motor for the servocontrol of the motor, the leakageflux adversely affects the revolution detector by causing errors indetecting revolutions and deteriorating the detecting accuracy.Accordingly, in some cases, a rotor shaft of a nonmagnetic material,such as a stainless steel, is employed in an electric motor with a DCbrake. However, nonmagnetic materials, including stainless steels, ingeneral are hard to work. It is difficult and costly to work such amaterial to form a rotor shaft. In addition, nonmagnetic materials arecomparatively expensive.

DISCLOSURE OF THE INVENTION

Accordingly, it is an object of the present invention to provide anelectric motor with a DC brake eliminating the above-mentioneddisadvantages, having a rotor shaft formed of two parts, namely, a partformed of a nonmagnetic material and a part formed of a free cuttingmetal, joined together by a recently developed pressure welding method.

According to the present invention, there is provided an electric motorwith a DC brake equipped on the rotor shaft thereof with a brakemechanism adapted to be disengaged when excited by DC current, in whichpart of the rotor shaft extending through and outward from the brakemechanism is formed of a nonmagnetic material and the other part, i.e.,the part extending within the rotor, is formed of a free cutting metalmaterial and both the parts of the rotor shaft are connected coaxiallyand integrally. The free cutting metal material applicable to the rotorshaft is a machine structural carbon steel or a chromium-molybdenumsteel, and the nonmagnetic material is a stainless steel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an electric motor with a DC brake,embodying the present invention, showing the general constitution of thesame;

FIG. 2 is a sectional view of an electric motor with a DC brakeembodying the present invention, showing the constitution of the same;and

FIG. 3 is a schematic diagram of another electric motor with a DC brakeembodying the present invention, showing the general constitution of thesame.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows the general constitution of a preferred embodiment of anelectric motor with a DC brake of the present invention equipped with aDC electromagnetic brake disposed in the front part of the motor.Referring to FIG. 1, a DC electromagnetic brake 10 and a electric motor12 have a common shaft 14. The rotor 16 of the motor 12 is mounted on apartial shaft 14a and the DC electromagnetic brake 10 is mounted on apartial shaft 14b. The partial shaft 14a is formed of a free cuttingmetal material while the partial shaft 14b is formed of a nonmagneticmaterial. The partial shafts 14a and 14b are connected together by theupsetting friction welding method. Therefore, the mechanical strength ofthe joint 18 is very high and partial shafts 14a and 14b are unitedperfectly. In the friction welding process, a free cutting metal rod anda nonmagnetic metal, each of a preselected diameter, are friction-weldedaxially, and then the rod thus welded is subjected to machiningaccording to the design specifications of the motor. Generally, sincethe length of the partial shaft 14a for the rotor 16 is greater than thelength of the partial shaft 14b for the DC electromagnetic brake 10, andhence the major part of the shaft 14 is formed of a free cutting metalmaterial, the shaft machining efficiency is improved. Furthermore, sincethe partial shaft 14b is formed of a nonmagnetic material and,therefore, the magnetic permeability of the same is small, the leakageflux leaking from the DC electromagnetic brake is not conducted outsidethrough the partial shaft 14b. Accordingly, a disadvantage of theconventional motor with a DC brake that powder of magnetic materialsadheres to a part of the rotor shaft extending frontward from the brakeand being connected to a driven device, for example the feed shaft of amachine tool, is eliminated completely. In case the material forming thepartial shaft 14a is a machine structural carbon steel and thenonmagnetic material forming the partial shaft 14b is a stainless steel,the use of the expensive stainless steel is reduced to the leastpossible quantity. Such a structure of the rotor shaft contributesgreatly to the reduction of the cost of the motor.

FIG. 2 is a sectional view of a motor with a DC brake as describedhereinbefore embodying the present invention. In this embodiment, a DCelectromagnetic brake 10 and a motor 12 are arranged along alongitudinal axis in a single unit housed in a housing 30 consisting ofa front housing 32, a central housing 34 and a rear housing 36. Thefront housing 32 contains the DC electromagnetic brake 10 and supports afront bearing 38. The central housing 34 contains the motor 12. The rearhousing 36 supports a rear bearing 40 coaxially with the front bearing38. The rotor shaft described in connection with FIG. 1, consisting ofthe partial shaft 14a for the rotor and the partial shaft 14b of anonmagnetic material penetrating through the DC electromagnetic brake 10is supported rotatably in the front and the rear bearings 38 and 40. Thepartial shafts 14a and 14b are united coaxially at a joint 18 byupsetting friction welding as mentioned earlier. The DC electromagneticbrake 10 includes a DC electromagnet 42, a brake disk 44, a brakingspring assembly 46, an external spline 48 axially slidably supportingthe brake disk 44, a movable brake pad 50a, and a fixed brake pad 50b.The DC electromagnet 42 is fixed to the fixed brake pad 50b and thefixed brake pad 50b is fixed to the central housing 34 by means ofsupporting bolts, respectively.

On the other hand, the motor 12 has a stator 54 fixed to the centralhousing 34 and a rotor core 56 secured to the partial shaft 14a.Windings 54a are mounted on the stator 54. When the windings 54a areenergized, a torque is generated in the rotor 16 formed of the partialshaft 14a and the rotor core 56.

In the motor with a DC brake of the above-mentioned constructionembodying the present invention, since no appreciable magnetic fluxflows into the partial shaft 14b even when the DC electromagnet 42 ofthe DC electromagnetic brake 10 is energized, part of the rotor shaft 14extending frontward from the front housing 32 is not magnetized.Accordingly, the connective capability of the projecting part of therotor shaft 14 is not subject to deterioration when used as a part formechanically connecting the motor to a driven body, not shown.

FIG. 3 shows the constitution of another embodiment of the presentinvention, in which an electric motor is provided with a DCelectromagnetic brake on the rear side thereof, and a tachometer fordetecting the output revolving rate of the motor, for example, anencoder, is attached to the rear end of the motor.

In this embodiment, the partial shaft 14a of the rotor 16 of the motor12 is formed, similarly to that of the above-mentioned embodiment, of afree cutting metal material while the partial shaft 14c of the DCelectromagnetic brake 10 disposed behind the partial shaft 14a is formedof a nonmagnetic material. Both partial shafts 14a and 14c are united toform an integral shaft 14 by the friction welding method at the junction18. Since a tachometer 20 is attached to the rear end of the nonmagneticpartial shaft 14c, the leakage flux of the magnetic force of the DCelectromagnetic brake 10 does not flow through the partial shaft 14c,and thereby the tachometer 20 is never exposed to the adverse effect ofthe leakage flux. Naturally, the second embodiment has the sameadvantages as those of the first embodiment.

We claim:
 1. A motor and brake apparatus comprising:(a) an electric motor; (b) a DC electromagnetic brake; (c) a power shaft extending through said motor and said brake and on which said motor and brake are mounted and projecting outwardly from said brake; (d) said shaft having a first portion of magnetic free-cutting metal extending through said motor and a second portion of non-magnetic metal extending through said brake and projecting outwardly therefrom for connecting the motor mechanically to a driven body, and said first and second shaft portions being united at a position axially to form an integral power shaft.
 2. An electric motor with a DC brake according to claim 1, wherein said partial shaft of the DC electromagnetic brake and said partial shaft of the rotor are united to form the integral rotor shaft by friction welding.
 3. An electric motor with a DC brake according to claim 2, wherein said partial shaft of the DC electromagnetic brake is formed of a stainless steel.
 4. An electric motor with a DC brake according to claim 1, wherein outward projecting part of the partial shaft of the DC electromagnetic brake forms a part for supporting a revolution detector. 